1. Field of the Invention
The present invention relates to an X-ray generator to be used for an X-ray inspection device for industrial or medical use, or various types of X-ray spectrometers or measuring devices using the diffraction or refraction of X-rays, and in particular to an X-ray generator in a system where X-rays are generated by making electrons collide with a target in a vacuumed atmosphere within an X-ray tube.
2. Description of Related Art
In X-ray generators, excluding special ones, a target and an electron source are placed within an X-ray tube that has been vacuumed, and electrons generated by the electron source are accelerated and made to collide with a target as an electron beam so that X-rays are generated. The generated X-rays are taken outside through an irradiation window that air tightly seals the inside of the X-ray tube from the outside.
Due to the difference between X-ray tubes in the means for holding a target, irradiating an electron beam, or taking out X-rays, the structure of the X-ray tube in the vicinity of the irradiation window is categorized as a transmission type or a reflection type as shown in the schematic diagrams of FIGS. 10 and 11.
In FIG. 10 showing a transmission-type X-ray tube, 101 is a target holder provided in an end portion of the X-ray tube, and 102 is a target where the target 102 is layered on the inside of an irradiation window 103, and thus formed so as to be integrated with the irradiation window 103. In this transmission-type X-ray tube, X-rays are generated from an X-ray generating point 102a, which is a spot on the target 102 to be irradiated with an electron beam, when the target 102 is irradiated with an electron beam B so that the X-rays are released to the outside through the irradiation window 103 mainly in the direction DT that is the same as the direction in which the electron beam B progresses.
Meanwhile, in FIG. 11 showing a reflection-type X-ray tube, 201 is a target holder, 202 is a target, and 203 is an irradiation window. X-rays are generated from an X-ray generating point 202a, which is a spot on the target 202 to be irradiated with an electron beam, when the target 202 is irradiated with an electron beam B so that the X-rays are released to the outside through the irradiation window 203 that makes contact with a target holder portion with an opening provided in the direction DR in which X-rays are taken out.
In both of the above-described transmission-type and reflection-type X-ray tubes, light metals such as Be or Al are used as the material of the irradiation windows 103 and 203.
Incidentally, the energy of the X-rays generated on the target is approximately 1% of the energy of the electron beam that strikes the target, and thus, the remaining 99% is converted to thermal energy. As a result, the temperature of the target 102 in the transmission-type X-ray tube shown in FIG. 10 becomes high, and therefore, the temperature of the irradiation window 103 that is integrated with the target 102 also becomes high.
Meanwhile, in the reflection-type X-ray tube shown in FIG. 11, the irradiation window 203 is not easily affected by the heat released from the target 202. However, the irradiation window 203 generates heat when the electron beam B reflected from the target 202 strikes it.
When the temperature of the irradiation window of the X-ray tube becomes high, it may cause various problems such as a gas released into the vacuumed atmosphere within the X-ray tube, a load on a brazed portion in a vacuum due to thermal stress, or a thermal effect in the case where the object to be inspected approaches the irradiation window from the air side.
Therefore, various means for suppressing the increase in the temperature of the irradiation window have been provided according to the prior art. For example, the irradiation window or its periphery are water-cooled or air-cooled, or such a structure is adopted that the target in a transmission-type X-ray tube makes close contact with a diamond, which is a material with excellent thermal conductivity, so that the heat is led to a heat radiator (see Patent Document 1). In a reflection-type X-ray tube, such a structure is adopted that a shield member is provided within the X-ray tube so as to prevent the electron beam reflected from the target from colliding with the irradiation window (see Patent Document 2).
In the case where a material with poor thermal conductivity is adopted as the material of the irradiation window, which is not usual, the point irradiated with the electron beam easily reaches the melting point in a vacuum.